Answer: There are a variety of ways. This actually gets into some complicated areas. I’ll go through some of the main issues; but just be aware that with each one of these, there can be details that are quite important that I’m not necessarily mentioning. But, if your parents are married and they are both U.S. citizens and at least one of those parents have lived in the United States or a territory of the United States before your birth, then you are likely already a U.S. citizen. Again, if married and if only one of your parents is a U.S. citizen and that parent has been physically present in the United States or a territory of the United States for a period of at least five years—at least two of those years after they were age 14—at some point prior to your birth, that would mean that you are already a U.S. citizen. Sometimes the physical presence requirements can be satisfied if your parents are employees of the U.S. government, worked for the armed forces or some international organizations as well.
Now, if your parents are not married and your mother is a U.S. citizen and has been physically present in the United States or an outlying possession of the United States for a continuous period of at least one year, then you are already a U.S. citizen. And if unmarried and your father is a U.S. citizen and that relationship is established meeting some requirements, then your father has to have met the physical requirements of at least five years in the United States or territory and at least two years of that after age 14, then you are already a U.S. citizen. And then there’s adoption as well. If you are adopted prior to your 16th birthday and live in the United States with your adoptive U.S. citizen parents, then you are a U.S. citizen.